(1) Field of the Invention
The present invention relates to a process for preparing cyclic urea derivatives. More particularly, the present invention relates to an industrially advantageous process for the preparation of a cyclic N,N'-dimethyl urea derivatives by reacting a cyclic urea compound with formaldehyde or paraformaldehyde.
(2) Description of the Related Art
Cyclic N,N'-dimethyl urea derivatives are important polar non-proton solvents and are widely used as medium for organic synthetic reactions, solvent for polymeric compounds, solvent for the extraction of aromatic hydrocarbons and unsaturated hydrocarbons in the petrolium refining, and the like. Thus, they are industrially very useful compounds.
The following processes are known as a conventional process for the preparation of a cyclic N,N'-dimethyl urea derivative.
(1) Process as described in U.S. Pat. No. 2,422,400
The process is comprised of two steps of (a) forming a methylol intermediate from 2-imidazolidinone and formaldehyde in an alkaline medium and isolating it and of (b) adjusting the pH of the reaction medium followed by the catalytic reduction.
(2) Process as described in Japanese Unexamined Patent Publication (Kokai) No. 52-71419
1,3-Dimethyl-2-imidazolidinone is prepared from 2-imidazolidinone and formaldehyde in one step by hydrogenation in a acidic medium of a pH of 3 to 5. In this publication, it is not specified and thus not clear what substance is used for acidifying the reaction mixture. However, in Example 1, there is described a process in which the reaction mixture is adjusted to pH of 3 by adding phosphoric acid. For the practice of the process, it is inevitably necessary to use a non-corrodible reaction apparatus, and in addition a salt which is difficult to be separated is produced by the neutralization of the reaction mixture after the completion of the reaction.
(3) Process as described in Japanese Examined Patent Publication (Kokoku) No. 60-3299
1,3-Bis-(hydroxymethyl)- or 1,3-bis-(alkoxymethyl)-2-imidazolidinone is reduced, in the presence of an acidic substance, using a hydrogenation catalyst. However, in order to prepare 1,3-bis-(hydroxymethyl)- or 1,3-bis-(alkoxymethyl)-2-imidazolidinone, it is necessary to prepare a methylol intermediate from 2-imidazolidinone and formaldehyde as in the process described in the above-mentioned U.S. Pat. No. 2,422,400, which indicates that the process substantially consists of two steps. Further, since it is necessary to carry out the catalytic reduction in the presence of an acidic substance at a pH of not higher than 3, a non-corrodible apparatus should be used and further a salt produced by neutralization after the completion of the reaction must be separated, as in the process described in the above-mentioned Japanese Unexamined Patent Publication No. 52-71419.
As mentioned above, the conventional processes for the preparation of cyclic N,N'-dimethyl urea derivatives are not satisfactory for the industrial purposes.